What's the Difference Between a Solar Eclipse, a Lunar Eclipse, and a New Moon?

This week, my family woke up at 4am to see the lunar eclipse. There’s nothing quite like watching the Moon turn to blood to start your day.

As I was watching the Moon fade to a dull red color, I started to think about the cosmic conjunction required to make an eclipse. That's why today I want to answer some questions I've received from Everyday Einstein readers and listeners about eclipses:.

Question #1: How does a solar eclipse work?

A solar eclipse happens when the moon gets directly between the Sun and the Earth. The Moon blocks part of the sunlight, causing a shadow to fall on the Earth. If you’re in an area covered by that shadow, you can see the eclipse.

Have you ever stood in a room that had multiple light sources? That's how you end up with multiple shadows. Since the Sun is quite a bit bigger than the Moon, the rays of light coming from the Sun can be thought of as multiple light sources as well. The ones coming straight at the Moon are completely blocked, creating the total eclipse area called the “umbra.”

However the ones coming at an angle towards the Moon are only partially blocked. These make a partial shadow area called the "penumbra." If you’re in the penumbra region, you only see a partial eclipse, because from your point of view, part of the Moon is still receiving light.

Question #2: How does a lunar eclipse work?

A lunar eclipse is similar to a solar eclipse. However in this case, the Earth gets in between the Sun and the Moon. The Earth’s shadow falls across the Moon, creating a lunar eclipse.

The reason the Moon looks red is that as the sunlight passes through the edges of the Earth’s atmosphere, its shorter wavelengths are scattered. (See the episode on Why Is the Sky Blue for more details). Because of this scattering, only the longer wavelengths of red light make it through to the Moon. (This is also why the Sun looks red as it sets).

Question #3: What’s the difference between a lunar eclipse and a new moon?

Well, one is an astronomical event, and the other is a book about vampires.

OK, corny jokes aside, a new moon occurs when the side of the Moon receiving sunlight is facing away from the Earth. Now you might be thinking, "Doesn’t that happen when the Moon is between the Sun and the Earth? If so, why isn’t there a solar eclipse every time there is a new moon?"

Well the fact is that every solar eclipse is a new moon, but not every new moon is a solar eclipse.

To understand this, imagine the Earth going around the Sun. Now, draw a line between the center of the Sun and the center of the Earth, we’ll call this line the “node line.”

Now imagine the Moon going around the Earth. Now take the Moon’s orbit and tilt it slightly. While this might be difficult to visualize, the result of this arrangement is that sometimes the Moon’s orbit goes above the node line and sometimes it goes below the node line.

Every time the Moon is at its closest point to the Sun, it’s a new moon. But if the Moon is above the node line when this happens, its shadow passes above the Earth. Likewise, if the Moon is below the node line, its shadow passes below the Earth.

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